The purpose of soap, of course, is to make something clean. Lye-based soap does that job very well. Lye soap is made of three primary ingredients: lye, oil/fat and water. The combination of these three items results in combination that works as a surfactant, that is, a material that works to break down and lift dirt, residue and grime from surfaces so that it can be rinsed away with water. If lye soap is made with too much lye, it can be harsh on skin and even produce a burned sensation and appearance on bare skin. If, however, lye soap is made with the correct proportion of lye to other ingredients, it reaches a pH balance that is strong enough to break down and remove the grime on skin without burning the skin.

The oil, fat or oil/fat combination that is used to make lye soap provides a secondary benefit. Once skin is cleansed, the oils in the soap provide moisture that can be absorbed by the pores of the skin. Different oils and fats have different benefits, but all provide the same basic benefits. They help to smooth the skin, and they help the skin to hold in moisture so that it does not become dry, chapped and irritated.

Not all lye soap is additive free, of course; almost any commercial bar soap is a lye soap, and almost all of those commercial lye soaps also contain some sort of fragrance or dye. However, if you want a very simple soap without additional ingredients such as fragrances or dye, which may irritate sensitive skin, then making your own lye soap is the way to go. Powdered lye is usually easy to find, and you can use any combination of oils and fats (though some will result in a softer soap and some in a harder soap, more like a bar soap), and when you have water handy. It's nice to have the option of a soap that will cleanse and moisturize without getting complicated.